Need advise about walking my 5mo female amstaff please

Hello I need advise or opinion please about my 5mo amstaff puppy during our walks .
Recently during our walk about a block in she suddenly starts biting her leash and pulling on it aggressively then she kinda loses it all together and thrashes around like a wild animal trying to get away and when I try to calm her down she gets worse and tries biting me after a minute or so she snaps out of it then returns to walking normal like nothing happened ..she also does this when my husband walks her as well .we are very concern about this we are not sure what sets her off both times she’s done  it while walking across the street in the crosswalk ..we’re afraid she will possibly get out of her harness and take off running or hurt us .does anyone know why she’s doing this and have advise as to what we can do to stop this behavior ??? Thank you